1. Field of the Invention
The present invention relates to a heated element sensor for detecting the flow rate of a fluid or other physical characteristics of a fluid. The invention uses a continuous waveform excitation to drive a single thermo-resistive element between two or more states.
2. Description of Prior Art
Heated element velocimeters have been known in the art. However, the techniques used to drive the heated element and to relate its temperature to the velocity or other properties of the fluid are significantly different from those described in the present invention.
U.S. Pat. No. 3,603,147 issued to Frank D. Doorman, uses a balancing bridge to supply power to self-heat a single temperature sensor. An amplifier coupled to the bridge produces a pulse output voltage which has a pulse width equal to the time it takes the temperature sensor to heat from ambient to a temperature that balances the bridge. The width of this pulsed output is determined by both the fluid temperature and its flow rate.
U.S Pat. No. 3,905,203 issued to Calvet et al, teaches a means for supplying a first pulse of electrical energy to a single heated thermo-resistive element followed at a preselected interval by at least a second pulse of electrical energy. The short pulses sample the electrical properties of the heated sensing element under two self-heating conditions. The invention requires either finding or extrapolating the ambient fluid temperature from the unexcited sensor. This temperature must then be used as a parameter in the determination of fluid velocity from the measurements.
U.S. Pat. No. 4,501,145 issued to Boegli et al, covers a Katta thermometer with automatic measurement of the time intervals required for a fixed amount of cooling. A weak current is provided to the sensor in order to assess the times required for the probe to reach the various temperature levels as it is equilibrated. From these times, kinetics of cooling at the probe are deduced and properties of fluid flow are computed.